Many laser gain media are capable of lasing at different wavelengths. By designing a laser cavity in a particular manner, a desired wavelength can be selected from the multiple possible lasing wavelengths by suppressing the gain at undesired wavelengths. Typically, a laser design is fixed for only one wavelength, and once the laser cavity is built, it is not a simple matter to adjust the laser to lase at a different wavelength.
For example, a simple method for selecting a particular wavelength is to use mirrors that only efficiently reflect the desired wavelength. For instance, dielectrically coated mirrors can be used that reflect only a narrow range of wavelengths. While this method allows one to tune a laser to a single wavelength or a narrow range of wavelengths, the method is not ideal, because it does not allow the operator to adjust or tune the wavelength once the laser has been built. In order to change the operating wavelength of the laser, the entire system must be rebuilt. In addition, if the gain medium supports lasing at wavelengths that are closely spaced, the mirrors may reflect more than the desired wavelength.
Another option for selecting a single wavelength is to use diffractive optics, such as diffraction gratings within the laser cavity, often in a Littrow configuration. The undesired wavelengths are spatially separated and blocked, thereby introducing substantial losses in the laser cavity at the undesired wavelengths and allowing gain only at the desired wavelength. Lasers using diffractive optics in this manner can be tuned by rotating the diffraction grating relative to the incoming beam. This solution is not without its problems, however. Alignment of the grating can be problematic, and the stability of the system can be lacking because the optical components must physically move to tune the laser.
This invention provides an electronically-adjustable system for selecting different wavelengths from the same laser cavity configuration without the need to change the laser cavity configuration by using a single acousto-optical device and a pair of optical reflectors inside the laser cavity, thereby creating a stable, wavelength-adjustable laser.